THE SACRAL AND COCCYGEAL VERTEBRA 63 



(for the reception of veins), which converge toward a single large, irregular aperture or several 

 small apertures at the posterior part of the body of each bone. The arch and processes pro- 

 jecting from it have, on the contrary, an exceedingly thick covering of compact tissue (Fig. 27). 

 The sacrum and coccyx consist mainly of spongy bone covered by a thin layer of com- 

 pact bone. 



FIG. 27. Bony structure of a lumbar vertebra. (Poirier and Charpy.) 



Development. Each vertebra is formed of four primary centres of ossification (Fig. 28), 

 one for each lamina and its processes, and two for the body. 1 Ossification commences in the 

 lamina 3 about the sixth week of fetal life, in the situation where the transverse processes afterward 

 project, the ossific granules spreading backward to the spine, forward into the pedicles, and out- 

 ward into the transverse and articular processes. Ossification in the body commences in the 

 middle of the cartilage about the eighth week by two closely approximated centres, which speedily 

 coalesce to form one central ossific point. According to some authors, ossification commences 

 in the lamina 1 only in the upper vertebrae i. e., in the cervical and upper thoracic. The first 

 ossific points in the lower vertebrae are those which are to form the body, the osseous centres 

 for the laminje appearing at a subsequent period. At birth these three pieces are entirely sepa- 

 rate. During the first year the lamina? become united behind, the union taking place first in 

 the lumbar vertebra? and then extending upward through the thoracic and lower cervical verte- 

 bra?. About the third year the body is joined to the arch on each side in such a manner that the 

 body is formed from the three original centres of ossification, the amount contributed by the 

 pedicles increasing in extent from below upward. Thus, the bodies of the sacral vertebra? are 

 formed almost entirely from the central nuclei; the bodies of the lumbar are formed laterallv 

 and behind by the pedicles; in the thoracic region the pedicles advance as far forward as the 

 articular depressions for the head of the ribs, forming these cavities of reception; and in the neck 

 the lateral portions of the bodies are formed entirely by the advance of the pedicles. The line 

 along which union takes place between the body and the neural arch is named neurocentral 

 suture. Before puberty no other changes occur, excepting a gradual increase in the growth 

 of these primary centres; the upper and under surfaces of the bodies and the ends of the transverse 

 and spinous processes being tipped with cartilage, in which ossific granules are not as yet de- 

 posited. At sixteen years (Fig. 30) three secondary centres appear, one for the tip of each trans- 

 verse process, and one for the extremity of the spinous process. In some of the lumbar vertebra?, 

 especially the first, second, and third, a second ossifying centre appears at the base of the spinous 

 process. At twenty-one years (Fig. 29) a thin, circular, epiphyseal plate of bone is formed 

 in the layer of cartilage situated on the upper and under surfaces of the body, the former being 

 the thicker of the two. These represent two additional secondary centres of ossification. All 

 these become joined, and the bone is completely formed between the twenty-fifth and thirtieth 

 year of life. 



Exceptions to this mode of development occur in the first, second, and seventh cervical, and 

 in the vertebrae of the lumbar region. 



Atlas (Fig. 31). The number of centres of ossification of the atlas is quite variable. It 

 may be developed from two, three, four, or five centres. The most frequent method is from 

 three centres. Two of these are destined for the two lateral or neural masses, the ossification 

 of which commences about the seventh week near the articular processes, and extends backward; 

 these portions of bone are separated from one another behind, at birth, by a narrow interval 

 filled in with cartilage. Between the third and fourth years they unite either directly or through 

 the medium of a separate centre developed in the cartilage in the median line. The anterior 



1 By many observers it is asserted that the bodies of the vertebra are developed from a single centre which 

 speedily becomes bilobed, so as to give the appearance of two nuclei; but that there are two centres, at all events 

 sometimes, is evidenced by the facts that the two halves of the body of the vertebra may remain distinct 

 throughout life, and be separated by a fissure through which a protrusion of the spinal membrane may take 

 place, constituting an anterior spina bifida. 



